This invention relates to a solid-state imaging device which is used for an optical character reader, a device, etc. More particularly, it relates to a driver circuit which can expand the dynamic range (the range of the quantity of light) of photodiodes in the solid-state imaging device.
A MOS type imaging device comprises a large number of photodiodes arrayed in the form of a matrix, and a circuit for detecting charges stored in the photodiodes. Each photodiode has a parasitic capacitance. When no light is incident on the diode, the parasitic capacitance is held at a potential substantially equal to the applied supply voltage. When light falls on the photodiode, a current flows, and stored charges in the parasitic capacitance are neutralized according to the magnitude of the current, with the result that the potential of the capacitance changes. By sensing the change of the potential, accordingly, the quantity of light having come into incidence on the photodiode can be converted into an electric signal corresponding to that quantity. In order to widen to the utmost the range of the quantity of light to be photoelectrically converted, i.e., the dynamic range of the photodiode, a voltage which is applied across terminals of the photodiode may be made great to increase the quantity of charges which are stored in the parasitic capacitance of the diode. Since, however, the voltage to be fed to the photodiode is applied through a switching element of a MOS transistor from a D.C. power source, it is limited by the threshold voltage of the transistor.